Previously expandable shafts for releasably gripping the inside diameter of the cores for winding webs, which shafts contained radially extending rams operated by fluid pressure, often failed because they were unable to: (a) seal statically, (b) prevent leakage when sliding seals were used, (c) prevent rupture when rubber inflatable bags were used, and/or (d) control the stability of the pressuring fluid.
Radially extending rams from such shafts to grip the inside of the cores of rolls of web, not only had to continuously grip radially outwardly but also were under large rapid varying stresses tangential to the shafts due to the torques and vibrations of the roll of web when changing its speed of rotation as in starting, accelerating, driving, braking, and stopping. This caused side wear in the piston rams or plungers, making annular seals between the relatively sliding parts, including O-rings, substantially useless, particularly at higher pressures, because of their leakage and/or seepage.
In a closed hydraulic liquid system of this type, less than 0.1% loss of fluid can prevent the maintenance of an operable pressure. The stability of the hydraulic liquid also is critical, since expansion or contraction from heat or cold changes the volume enough, respectively, either: to activate the rams, or to reduce the volume of the liquid below the capacity of the system so that pressurizing is impossible.
Gas or air operated fluid systems which use inflatable bags have proved to be undesirable under heavy loads because they are limited to the amount of initial pressure they need or have available, and the gas is further compressible under such heavy loads, whereas a liquid is relatively noncompressible. Furthermore, such bags tend to develop leaks under heavy loads because of their limited strength and because of their wear through vibration and their contact with the rams and/or leaves against which the bags press to grip the cores during operation.
Because of these variations in fluid pressure during operation, leakage often occurs, particularly when relatively long periods of lock-up or gripping of the shafts against cores is required, i.e. up to about 24 hours. Therefore a continuous or constant supply of fluid pressure from an outside source is required, which means a movable joint connection to the shaft which further causes leakage, and which means the additional expense for and complexity of an outside fluid pressure system.
Still further, the amount of pressure which could be maintained in such shafts, either by means of a bag expanded against the gripping pistons, or by direct pressure against such gripping rams or pistons, was rarely above about 200 psi, and usually was between about 100 and 200 psi, thus limiting the amount of grip which could be applied to a core to be rotated on such shafts.
Accordingly, the purpose of this invention is to eliminate these difficulties.